tests: fix subprocess cleanup when dying

tests: fix subprocess cleanup when dying

We need to kill the tool subprocess when bailing out during tests
everytime, not only when an error during test_tool_wait() occurs.

Also: we need to wait() for killed processes too as otherwise we may
end up calling delete_module() before the process actually stops using
gpio-mockup giving us a nasty error message from libkmod and leaving
the module in memory.

Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>